1. Field of the Invention
The present disclosure relates to a light emitting device that includes a light emitting element together with a phosphor.
2. Description of the Related Art
In recent years, light emitting diodes (hereinafter also referred to as LEDs), which provide substantial energy savings, are widely used as light emitting elements. In addition, a light emitting device includes a semiconductor light emitting element containing gallium nitride (GaN) and a yellow phosphor which is excited by light from this light emitting element and emits yellow luminescent radiation. This light emitting device can emit white range light by color mixture, and is used for illumination.
Also, in the technical field of illumination, in order to perform stage light rendering or to light up structure such as building and bridge, light rendering is used by various colors of light as well as white light.
LEDs are already known which emit monochromatic red light, green light and blue light corresponding to the primary colors of light. However, in the case where these monochromatic LEDs are used together, the voltages for driving these LEDs are different since the LEDs which emit monochromatic red light, green light and blue light are formed of different types of semiconductors. For this reason, if a light emitting device is constructed of the LEDs which are driven at different voltages, it is necessary to take their different characteristics of the LEDs into consideration. Accordingly, the construction or control means of the light emitting device will be very complicated. As a result, the device itself, and its operational cost may be expensive.
On the other hand, it is conceivable such a monochromatic LED is replaced by a monochromatic light emitting device which includes a light emitting element and a phosphor which is excited light from this light emitting element. For example, International Publication No. WO 2009-099,211 A discloses a light emitting device which includes a light emitting element that emits light from ultraviolet to blue light range, and a phosphor that absorbs the light from this light emitting element and emits a different wavelength of light (wavelength conversion) whereby emitting red light.
For example, as a light emitting device which includes a light emitting element and a phosphor, and emits red light, in the case where a light emitting device includes a light emitting element that emits light containing ultraviolet radiation, and a phosphor that is excited by the ultraviolet radiation, there is a possibility that some members (e.g., resin) of the light emitting device may be deteriorated by the ultraviolet radiation.
In particular, in the case where this type of light emitting device is small, the amount of the phosphor may not be enough to sufficiently absorb light from the light emitting element since the space of the small light emitting device is limited. If the amount of the phosphor is enough, a part of light from the light emitting element is directly extracted from the light emitting device, that is, the light emitting device emits unintended color of light. As a result, there is a problem that the expected color purity of the monochromatic red light emitting device is not achieved.
For example, the phosphor disclosed in WO 2009-099,211 A has a relatively high reflectance at a wavelength of 450 nm corresponding to the peak wavelength of excitation light from the light emitting element. For this reason, a relatively large amount of the phosphor is required for sufficient absorption of the light for exciting the phosphor. On the other hand, in particular, in the case where a light emitting device is small, the space of the small light emitting device is limited for accommodating the phosphor, that is, a sufficient amount of the phosphor for emitting red light may not be accommodated. In this case, a part of light from the light emitting element which is not absorbed by the red phosphor is extracted from the light emitting device. In other words, a part of blue light is extracted from the light emitting device. As a result, the red color purity of the light emitting device may be insufficient for a red monochromatic light emitting device.
The present invention is devised in light of the disadvantageous features as described above. It is one object of the present invention is to provide a light emitting device capable of emitting monochromatic red light with high color purity.